1. Field of the Invention:
The present invention relates to an apparatus for detecting a reference position of a servo-controlled member such as a throttle valve of an internal combustion engine, and more particularly to a servo-driving apparatus which includes a DC motor controlling the position of the movable member, wherein a reference position of the movable member is detected by a reference position detecting apparatus having a novel constitution.
2. Description of the Prior Art:
A known prior art, e.g. Unexamined Japanese Patent Application No. SHO 58-10131, has disclosed a method of detecting a reference position of the throttle valve, in which a fully opened position or a completely closed position of the throttle valve is repeatedly detected to obtain a reference position of the throttle valve through learning process on the basis of the detected throttle valve opening angle. In more detail, this prior art has introduced a specially provided switch for detecting a completely closed position of the throttle valve. This switch is cooperatively associated with a conventional throttle valve opening sensor which generates a linear output in response to the detected opening degree of the throttle valve. The following is the reason why such a special switch is required.
The throttle valve opening sensor is normally accurate in the detection of how much angle the throttle valve is opened or closed, but is incapable of accurately detecting how completely the throttle valve is closed. Because, a significant amount of installation error is inevitable in the assembling of the throttle opening degree sensor onto the throttle valve.
Therefore, above special switch (referred to as a complete close switch, hereinafter) is utilized to compensate such a deficiency of the throttle valve opening sensor. That is, in operation, an output signal of the throttle valve opening sensor, generated when the complete close switch is depressed by the throttle valve at the completely closed position, is memorized as a reference position representing a complete close position of the throttle valve.
However, the installation of the complete close switch is never free from the error. As the computer-based engine control has been advanced, an accuracy in the detection of the complete close position has been becoming important and therefore will be more severely required in the future. For this reason, even if an installation error of the complete close switch remains within a narrow range, it may not be acceptable for the present-day and future precise engine control. It is therefore feared that small installation error of the complete close switch will give an adverse affect to an overall control of the throttle valve opening degree.
On the other hand, there has been conventionally known a method of detecting a lock current of the DC motor. This lock current, flowing through a resistance provided in series with the DC motor, is detected in such a case where the driving motor is forcibly stopped by an obstacle or the like. The circuit for detecting this lock current, however, tends to be complicated in construction and large in size because of necessity of providing at least the following three components, i.e. a resistance detecting lock current, an amplifier circuit for amplifying an output voltage obtained from the resistance, and a conversion circuit for converting the analogue output of the amplifier circuit into a digital signal.